The techniques of moulding that are standard for reinforced thermosetting resins consist of subjecting the resin associated in a pasty form with the reinforcement to the conjugate action of pressure and heat inside a mould. In the mould, cross-linking is thus caused to take place. This hardens the resin and gives to it the shape of the mould in a definitive manner.
It then remains to eject the composite piece thus formed from the mould. A distinction can be drawn between various types of technique, linked notably to the feed device for the mould and to the functioning thereof.
In the case of compression moulding, the feed system is of the piston type and the mould is constituted of two half-moulds. One half fits by a vertical translatory motion into the other, in which the resin and the reinforcement are present.
In the case of injection moulding, moulding devices are used based upon the same principle as those used for the moulding of thermoplastics, but adapted to thermosetting materials. The resin, associated with the reinforcement, is introduced into a cylinder provided with an Archimedean screw. The material is subsequently injected in defined proportions into the mould, which then opens out into two parts for the moulding to be ejected.
In each of these techniques, the efforts of the industry are directed towards reducing the cycle time for the moulding, which now is of the order of one minute, from the introduction into the feed system for the mould to the ejection of the final composite. But efforts are also being applied to the optimization of the steps for the production and processing of the starting constituents, Which precede the moulding proper, notably of the step known as the mixing step. This step consists of incorporating the reinforcement into the resin. The aim is to achieve this without the quality of the composite being adversely affected thereby.
This composite must, in effect, combine mechanical and aesthetic properties in a perfectly reproducible manner. It is the function the glass thread reinforcement to give mechanical properties to the composite material, and notably its mechanical characteristics in bending and/or when subjected to shock or impact.
Furthermore, the external appearance of the component must avoid defects such as problems of planeity, flash, visible fibres, or dimensional problems. These problems can result, for instance, from incomplete filling of the mould in the case of injection moulding, or from a too pronounced "shrinkage" phenomenon.
All these requirements mean, notably, that efforts are made to achieve an optimum distribution between reinforcement and resin.
In known manner, any moulding operation is preceded by the preparation of that which hereinafter will be termed impregnation paste, that is to say the mixture of the thermosetting resins, such as an unsaturated polyesters, with the usual additives, such as for example fillers, a catalyst, a stabilizer, and a demoulding agent.
On the other hand, the thread reinforcement is obtained by chopping one or more basic threads, each composed of a large number of continuous filaments. These filaments are obtained by mechanical drawing of the molten glass flowing from a plurality of orifices disposed on the bottom of a bushing, generally heated by Joule effect. These filaments are coated, and assembled without twisting into at least one thread before being wound into the form of a coil on a mandrel.
A collection of threads may be produced by simultaneously unreeling several coils, then assembling them together in parallel and winding them onto a mandrel to form a stand. This strand is then termed "assembled roving", the strand subsequently being chopped into pieces of varying length.
It then remains to incorporate, into the impregnation paste previously defined, the reinforcement in the form of these pieces of assembled roving, in order to manufacture what is known as "prepreg". This operation takes place in a mixer, in a manner known to the person skilled in the art, either by a humid process or by a dry process. Afterwards, this prepreg is allowed to "ripen" for a given period of time before it can be used for supplying the moulds.
In this way composite products are obtained, the quality of which may be judged satisfactory overall. So many parameters are involved that the specialist can optimize the mechanical and/or aesthetic characteristics that are more particularly desired.
However, it still remains that the production phase for these assembled roving strands can be broken down into several steps, of a discontinuous nature, and requires operations such as the unwinding-rewinding step, operations which require both time and space. It is thus necessary to provide for storage room, and for sufficient equipment for carrying out the winding.
In fact, by choosing the reinforcement in the form of assembled roving, the person skilled in the art had arrived at a kind of compromise, since the reinforcement had to comply with fundamentally contradictory criteria.
First of all, since the reinforcement was incorporated into the impregnation paste in the form of short lengths, the make-up of the roving strand must be such that, when it is cut, the appearance of isolated filaments which become electrostatically charged and foul the cutting apparatus shall be as small as possible. The assembled roving strands, by reason of the fact that they are made of threads possessing their own cohesion and of small size considered one by one, satisfy this criterion well.
Subsequently, chopped strands obtained are mixed with the impregnation paste and, during this step, the reinforcement should disperse in optimum manner. Here again, the assembled roving eminently satisfies this requirement. In fact, the threads have no cohesion whatever among themselves within the strand and they separate at the end of the chopping operation. This first division prepares and promotes the later dispersion of the reinforcement within the body of the resin in the form of filaments. This shows how much the presentation of the reinforcement in the strand and the coating substance used play a major part in reconciling contradictory requirements. Since the assembled roving best complies with these requirements, the person skilled in the art, in spite of the disadvantages which this product possesses in other respects, has concentrated on improving the coating of the threads of which it is made.